1. Field of the Invention
The present invention relates to a positive-negative pulse type high frequency switching power supply unit which applies positive and negative pulse voltages in a high frequency to a load, and more particularly to a positive-negative pulse type high frequency switching power supply unit which can be utilized widely to various apparatus including an atmospheric plasma generator, a DC/DC converter, a battery charger, a high frequency corona processor, an ozone generator, an invertor for a motor, a sputtering apparatus, a lamp light source, a soldering apparatus and a discharging unit.
2. Description of the Related Art
A positive-negative pulse type high voltage power supply unit is disclosed in Japanese Patent Laid-Open No. Hei 9-172787. The positive-negative pulse type high voltage power supply unit is constructed in the following manner to assure good rising and falling characteristics of high positive and negative pulse voltages. It is to be noted that, in the following paragraph, a reference symbol same as that used in Japanese Patent Laid-Open No. Hei 9-172787 is added in a parenthesis to each component in order to facilitate recognition.
In particular, a first switching element (SW1), a second switching element (SW2) and a third switching element (SW3) are connected in series between a positive voltage generation section (+E) and the ground, and a load (R) is connected between a node between the first switching element (SW1) and the second switching element (SW2) and the ground while a fourth switching element (SW4) is connected to a node between a negative voltage generation section (-E) and the node between the first switching element (SW1) and the second switching element (SW2) connected to the load (R). The first switching element (SW1) is first turned on to apply a positive voltage to the load (R) and then the second switching element (SW2) is turned on to discharge the positive charge of the load (R) by the circuit to the ground through a diode (D3) connected in parallel to the third switching element (SW3). Then, the fourth switching element (SW4) is turned on to apply a negative voltage to the load (R) and then the third switching element (SW3) is turned on to discharge the negative charge of the load (R) by the circuit to the load R through a diode (D2) connected in series to the second switching element (SW2).
The conventional positive-negative pulse type high voltage power supply unit is advantageous in that, because the voltage values of the high positive-negative pulse voltages are variable individually, where the power supply unit is applied as a power supply for a discharger, the ion balance can be adjusted. However, the power supply unit has a problem that it requires positive and negative power supplies (the positive voltage generation section +E and the negative voltage generation section -E) as power supplies to be supplied to the switching elements.
Another positive-negative pulse type high voltage power supply unit is disclosed in Japanese Patent Publication No. Hei 7-57100 wherein four semiconductor elements are connected in an H-bridge configuration and are switched alternately between positive and negative voltages at the ratio of 50/50%. Further, the power supply unit adopts a PDM (pulse density modulation) system wherein a rest period is provided between pulses in the waveform of an output pulse signal at the ratio of 50/50% between the positive and the negative voltages as seen in FIG. 12 to obtain high voltage pulses resonant with the output side.
The positive-negative pulse type high voltage power supply unit just described, however, is disadvantageous in that it is complicated in circuit configuration and is difficult in modulation control and besides a wide modulation width cannot be assured.
Also such a further positive-negative pulse type high voltage power supply unit as shown in FIG. 13 is conventionally known. Referring to FIG. 13, the positive-negative pulse type high voltage power supply unit shown employs an H-bridge switching circuit wherein four first, second, third and fourth semiconductor switching elements SW1, SW2, SW3 and SW4 are connected in an H-bridge connection and diodes D1, D2, D3 and D4 are connected in parallel to the semiconductor switching elements SW1, SW2, SW3 and SW4, respectively. The positive-negative pulse type high voltage power supply unit successively and repetitively performs a switching operation in a pattern of four on/off combinations of stages 1, 2, 3 and 4 listed in Table 1 given below:
TABLE 1 ______________________________________ 1 2 3 4 ______________________________________ SW1 ON OFF OFF OFF SW2 OFF OFF ON OFF SW3 ON OFF OFF OFF SW4 OFF OFF ON OFF ______________________________________
In particular, referring to FIG. 13, the four semiconductor switching elements SW1, SW2, SW3 and SW4 are all in an off state first (the load is off at the opposite ends thereof). Then, if signals are inputted simultaneously to the gates of the semiconductor switching elements SW1 and SW3, then current flows in the direction indicated by an arrow mark I1 in FIG. 13 and charges the load. Thereafter, the gate signals to the semiconductor switching elements SW1 and SW3 are stopped. However, the charge accumulated in the load remains in the load. Then, if signals are applied simultaneously to the gates of the semiconductor switching elements SW2 and SW4, then current flows in the direction indicated by another arrow mark I2 in FIG. 13 and discharges the load. Thereafter, the gate signals to the semiconductor switching elements SW2 and SW4 are stopped. However, the charge of the opposite polarity accumulated in the load remains in the load.
Accordingly, even if the gate signals to the semiconductor switching elements SW1 and SW3 or SW2 and SW4 stop, the output pulse signal does not fall immediately, but the output pulse waveform is influenced by a floating capacitance and a leakage inductance of the load such that, where the load is light and is a capacitive load, a pulse is extended to a rising edge of a next pulse as seen from the waveform (A) of FIG. 14, but where the load is a reactive load, each pulse is distorted at the leading and trailing edges thereof as seen from the waveform (B) of FIG. 14. Consequently, accurate PWM (Pulse Width Modulation) cannot be achieved.
Further, a conventional high-capacity high frequency power supply unit has following problems because it usually employs a self-excited invertor which includes a series resonance or parallel resonance type converter to cause resonation to occur.
1 In order to stabilize the output of the self-excited invertor, such a controlling method as to supply dc current to a transformer using a circuit such as a magnetic amplifier to cause leakage to occur is used, and therefore, it is difficult to design the high-capacity high frequency power supply unit.
(2) Where a self-excited oscillation system is employed, the frequency to be applied to a transformer is varied for frequency control, and therefore, a large amount of heat is generated.
(3) Where a self-excited invertor is used, output control of 0 to 100% cannot be achieved.